This invention relates to sorbitol derivatives useful as nucleators and clarifiers in polyolefin resin compositions and to polyolefin resin compositions in which such derivatives are included, thus imparting improved transparency and improved crystallization response.
Polyolefins such as polyethylene and polypropylene have found extensive use as packaging materials and containers in the form of films, sheets or hollow articles. However, since polyolefins tend to have poor transparency, they cannot fully meet the demand for packaging materials or containers which permit their contents to be viewed from the outside.
It is generally known to use nucleators and clarifiers to reduce product molding cycle times by allowing the polyolefins to which they are added to be crystallized at higher temperatures during injection and blow molding operations. Polyolefins containing such additives require less time in the mold for cooling and the finished product has smaller crystals, resulting in decreased light scattering and improved clarity.
The preparation and purification of dibenzylidene sorbitol is disclosed in U.S. Pat. No. 3,721,682, U.S. Pat. No. 4,131,612 and U.S. Pat. No. 4,267,110; these references also disclose uses which include gelling of organic liquids and clarifying polyolefin resins. Dibenzylidene sorbitol has been proposed as an additive (Japanese Patent Application No. 94424/1974) to improve transparency, but this additive had reported compatibility limitations with polyolefin resins. The addition of a small amount of tribenzylidene sorbitol to dibenzylidene sorbitol improved compatibility with polyolefin resins (U.S. Pat. No. 4,267,110), but it was later reported that it tended slightly to bleed out onto the surface of the resulting molded article, and therefore the surface of the molded article tended to collect static charges and become contaminated.
Polypropylene compositions having improved transparency and being substantially free from bleeding were reported in U.S. Pat. No. 4,314,039 based on the use of 1,3-2,4-di(alkylbenzylidene) sorbitol in which each alkyl group has (the same) 2 to 18 carbon atoms; the position of the alkyl group was reported not to be critical.
There are several commercial nucleators based on sorbitol which are useful in polyolefins including: 1,3-2,4-di(benzylidene)-D-sorbitol (Millad 3905, Milliken Chemical Co.); 1,3-2,4-di-(4-tolylidene)-D-sorbitol (Millad 3940, Milliken Chemical Co.); 1,3-2,4-di-(4-ethylbenzylidene)-D-sorbitol (NC-4, Mitsui Petrochemical Industries, Ltd.).
Processes for preparing dibenzylidene sorbitol and xylitol derivatives are described in U.S. Pat. No. 4,429,140 and U.S. Pat. No. 4,562,265. The substituted benzaldehydes disclosed as useful in the processes for reaction with sorbitol or xylitol include lower alkyl or alkoxy, halogen or nitro group as well as other aromatic aldehydes.
U.S. Pat. No. 4,371,645 discloses various dibenzylidene sorbitol derivatives for use in polyolefin plastic compositions to improve transparency. However, it is taught that the derivatives must be disubstituted with chlorine or bromine. The presence of the halogen is said to significantly improve transparency.
Polyolefin resin compositions employing a variety of dibenzylidene sorbitol derivatives for improved transparency are disclosed in U.S. Pat. No. 4,483,952. The polyolefins include polyethylene and polypropylene as well as copolymers of propylene with a small amount of ethylene. The two substituting groups on the dibenzylidene sorbitol are disclosed as different from each other and in the case where they are alkyl groups, they are limited to three carbon atoms or less. The classes of derivatives are said to have very good compatibility with polyolefin resins, thereby avoiding bleeding out or static charge build-up on the surface of articles molded from the composition.
Another reference which discloses disubstitution by a halogen, in this instance fluorine, is U.S. Pat. No. 4,808,650. The fluorine must be present in the structure in the meta position, although additional fluorine can be present in the para position thus resulting in a tetrafluoro derivative. The latter is said to be particularly effective at concentrations of 0.4 to 0.6% by weight.
Polyolefin compositions with both high clarity and resistance to oxidative degradation are disclosed in U.S. Pat. No. 4,845,137. The improvement is said to result from the presence of a sulfur-containing substituent on at least one of the two benzylidene rings of the dibenzylidene sorbitol. The preferred additive contains a lower alkyl thio group on each ring.
Another process patent, U.S. Pat. No. 4,902,807 discloses a process for batchwise production of substituted or unsubstituted dibenzylidene sorbitol or xylitol based on slowly feeding the primary reactants (polyhydric alcohol and benzaldehyde compound(s)) to the reactor. The primary reactants, and the benzaldehyde compound in particular (col. 3, lines 34-45), are not different from those disclosed in the above references. Various process benefits are attributed to the reaction control features.
U.S. Pat. No. 4,996,334 discloses dibenzylidenesorbitol derivatives having at least one nuclear substituent selected from an ester group such as --COOR.sup.1 and an amido group represented by --CONR.sub.2 R.sup.3 ; R.sup.1,R.sup.2 and R.sup.3 are defined elements or groups. The derivatives are disclosed as useful for heightening transparency and rigidity of resins (col. 1, lines 9-11) although no evidence is provided. The reference disclosure and examples are devoted exclusively to the use of the disclosed compounds as gelatinizers for oils, organic solvents and other fluids.
Dibenzylidenesorbitol derivatives are disclosed in U.S. Pat. No. 5,001,176 for use in combination with a cyclodextrin compound to produce crystalline polyolefin articles with improved transparency and improved odor. The reference does not suggest that the dibenzylidenesorbitol derivatives exemplified in column 3 are novel; it is the use of the cyclodextrin to reduce the odor of the composition, which appears to be the advance.
Mixtures of various dibenzylidene sorbitol and xylitol derivatives useful as nucleating agents for crystalline resin compositions are disclosed in U.S. Pat. No. 5,015,684. The mixtures include dibenzylidene sorbitol and xylitol per se as well as derivatives wherein the rings are substituted with 2 or 3 methyl groups (columns 3 and 4).
More highly substituted benzylidene sorbitols containing C.sub.1 -C.sub.4 dialkyl (or up to a 5 carbon atom carbocyclic ring) substitution on each benzylidene group are disclosed in U.S. Pat. No. 5,049,605. Polymeric compositions containing the additives are said to have improved taste and odor properties.
U.S. Pat. No. 5,023,354 describes an improved process for preparing high purity diacetals in the absence of organic solvents and in an aqueous medium. The advance appears to be the process conditions which are particularly specified; the use of the resulting product for clarifying and stabilizing polyolefins is mentioned in passing (abstract and col. 2, lines 4-8), and it is not suggested that the specific derivatives are novel.
U.S. Pat. No. 5,041,310 discloses a process for coating individual semi-crystalline polymer particles with liquid additive compositions including polyolefin clarifiers, such as sorbitol derivatives which derivatives also function as gelling agents. Several gelling agents are disclosed at col. 3, lines 47-62 including the ring substituted derivatives 4,4'-dimethyl dibenzylidene sorbitol and 4,4'-bis(methylthio) dibenzylidene sorbitol.
While relevant art is available, it is apparent that structural variations lead to differing results; no underlying theory is available which allows for predictability of individual compound performance.